This invention relates to methods of forming vertical resonant cavity optical structures upon a substrate. More particularly, this invention relates to a method for precisely varying at least one characteristic of such structures between individual laterally displaced structures on the substrate. The characteristics include wavelength of emission or detection, bandgap energy, thickness of layers within the structures, chemical composition of the layers within the structures, and lateral variation of the amounts of different elements within a layer.
The structures are vertical-cavity resonance optoelectronic devices and include vertical-cavity surface-emitting lasers (VCSELs), resonance-cavity photodetectors (RCPDs) and Fabry-Perot cavity modulators (FPCMs) that are useful in optical communications and sensing. A 1- or 2-dimensional (2D) device array emitting, detecting, or modulating light at different wavelengths enables many unique applications. For instance, a VCSEL array with different wavelengths can be used for wavelength-division multiplexing (WDM) fiber-optic communication systems. Different wavelengths from different array elements can be coupled in a single fiber for transmission over a distance. Each wavelength can be differently encoded and a de-multiplexing system at the receiving end can separate the different channels. Such a scheme can greatly enhance the transmission capacity. A VCSEL array WDM system is ideal for campus-wide short-haul applications. In addition, such an array has been demonstrated to be very useful for reconfigurable multiple chip module free-space interconnects. An array of RCPDs sensitive to different wavelengths enables a compact and integrated multi-channel spectroscopic analysis microsystem for quantitative fast parallel optical sensing. The detectors consist of a closely-spaced resonance-cavity detector array. Each element in the array is only sensitive to a specific wavelength of the broadband irradiation. Therefore, the array is equivalent to an integration of a spectrometer and a detector array. An array of FPCMs also offers a unique capability for multiple wavelength communication systems.
The central part of the resonance cavity devices is the optical cavity embedded between two distributed Bragg reflector (DBR) mirrors. The wavelength of emission, detection, or modulation is dictated by the Fabry-Perot mode of the cavity, which is determined in turn by the optical thickness (the product of layer thickness with the refractive index) of the cavity. The operating wavelength will be changed if the thickness of the optical cavity is changed.
It would be desirable to be able to fabricate an array of resonant-cavity devices from a single growth with a reliable manufacturing process. Such a process would require lateral variation of the layer thickness and alloy composition in these complex, multi-layer resonant structures. Prior to the invention of the process disclosed herein, no such process was known to exist.
Selective area growth (SAG) by metalorganic vapor phase epitaxy (MOVPE) has proved to be a viable technique for the lateral definition of the thickness, composition, and bandgap energy of semiconductor material at different regions of the same wafer. The substrate (or base epitaxial structure) can be partially masked by dielectric materials, such as SiO.sub.2, SiN.sub.x, and SiON.sub.x, and perfect selectivity of the growth (no growth occurs on the mask material) can be achieved for many III-V materials under certain growth conditions. The growth selectivity redistributes the flux of reactant gases in the MOVPE growth, and diverts the metalorganic materials from the mask region into the open area. Different degrees of enhancement or modulation of the thickness and alloy composition can be achieved on the same wafer nearby the different mask patterns by varying the dimensions of the masked area. SAG technique has been used for optoelectronic/photonic integration of laser-modulators, multiple-wavelength (ID) laser-passive waveguides, laser-detectors, and other similar linear stripe structures. See for example, U.S. Pat. Nos. 5,704,975; 5,728,215; 5,770,466; and 5,828,085.
Review of the literature indicates that little has been done on the demonstration of multiple wavelength RCPDs and FPCMs. However, several approaches have been reported in making WDM VCSELs. Chang-Hasnain et al reported a successful fabrication of a multi-element VCSEL array with wavelength space .about.3 nm between the neighboring elements in the array. C. J. Chang-Hasnain, J. P. Harbison, C. E. Zah, M. W. Maeda, L. T. Florez, N. G. Stoffel, and T. P. Lee, "Multiple wavelength tunable surface-emitting laser arrays," IEEE J. Quant. Electron. vol. 27, pp. 1368, 1991. Their approach was to use the inherent nonuniformity of the beam flux profile in molecular beam epitaxy (MBE) growth. Substrate rotation, which is used to average out the material nonuniformity, was stopped during the growth of the cavity. Therefore, different cavity wavelengths were achieved using the tapered cavity thickness. Since the degree of the taper critically depends on the substrate position relative to the sources, this approach has problems with the producibility of the absolute wavelength and wavelength spacing.
The second approach is to etch grooves on the backside of the substrate for MBE growth. W. Yuen, G. S. Li, and D. J. Chang-Hasnain, "Multiple-wavelength vertical-cavity surface-emitting laser arrays with a record wavelength span," IEEE Photon. Technol. Lett. vol. 8, pp. 4-7, 1996. The substrate is then mounted to a Mo holder with In solders. The differential thermal contact creates a temperature profile near the edge of the etched groove. When the growth temperature is high enough, the material in the cavity region creates a thickness profile due to differing degrees of thermal desorption. Therefore, different wavelengths can be achieved.
The third approach is to pattern and etch the substrate with different sizes (and depths) of mesas. F. Koyama, T. Mukaihara, Y. Hayashi, N. Ohnoki, N. Hatori and K. Iga, "Wavelength control of vertical-cavity surface-emitting lasers by using nonplanar MOCVD," IEEE Photon Technol. Lett. vol. 7, pp. 10-12, 1995, and G. G. Ortiz, J. Cheng, S. Z. Sun, H. Q. Hou, and B. E. Hammons, "Monolithic, multiple wavelength vertical-cavity surface-emitting laser arrays by surface-controlled MOCVD growth rate enhancement and reduction," IEEE Photon. Technol. Lett. vol. 9, pp. 1066-1068, 1997. The topographical difference causes differences in the surface diffusion process and, therefore, creates a cavity thickness variation for different lasing wavelengths. Wavelength difference was obtained from different mesas. This approach lacks the accuracy of the wavelength control and introduces a topographical profile, making it difficult to process. Another major problem with these approaches is the narrow operable temperature range. Since all the lasers with a large wavelength span share the same active region of the quantum wells, the optimum operating temperature for each element is very different. This results in a very narrow temperature region of the quantum wells, the optimum operating temperature for each element is very different. This results in a very narrow temperature region within which the operation of all the lasers can be achieved.
However, WDM VCSELs meeting the specification of the wavelength accuracy, spacing, and device performance are becoming more desirable for fiber-optic communications than ever before. For example, a LAN project sponsored by the U.S. Government required a 1.times.4 array with wavelength spacing of 15 nm between the neighboring channels. Currently, the laser array is rather inelegantly achieved by using 4 discrete devices from 4 individually optimized runs. Clearly, an integrated device with suitable performance would be superior if one could be made. For a practical application of WDM VCSELs, the critical issues include a predefined wavelength spacing, absolute wavelength accuracy for each element, and uniformity of the device performance. The current state of the art of technologies cannot satisfy these demands. It would be very desirable if the gain wavelength for each individual element of such a laser array "tracks" the laser wavelength so that a uniform device performance over a range of wavelengths can be achieved. None of the prior art above teaches a method to successfully form these individual elements in 1 or 2 dimensional arrays.